Field
This disclosure relates to methods of servicing a wellbore. More specifically, it relates to methods of treating lost circulation.
Background
Natural resources such as gas, oil, and water residing in a subterranean formation or zone are usually recovered by drilling a wellbore down to the subterranean formation while circulating a drilling fluid in the wellbore, which may be circulated downward through the interior of the drill pipe, out the drill bit, and upward through an annulus between the drill pipe and the wellbore wall. After termination of drilling operations, a string of pipe, e.g., casing, may be run in the wellbore. Circulation of drilling fluid may be resumed downward through the interior of the casing and upward through an annulus between the casing and the wellbore wall. Next, primary cementing is typically performed whereby a cement slurry is placed in the casing-wellbore annulus and permitted to set into a hard mass (i.e., sheath) to thereby attach the string of pipe to the walls of the wellbore and seal the annulus. Subsequent secondary cementing operations may also be performed.
Subsequently, oil or gas residing in the subterranean formation may be recovered by driving the fluid into the well using, for example, a pressure gradient that exists between the formation and the wellbore, the force of gravity, displacement of the fluid using a pump or the force of another fluid injected into the well or an adjacent well. The production of the fluid in the formation may be increased by hydraulically fracturing the formation. That is, a viscous fracturing fluid may pumped down the casing to the formation at a rate and a pressure sufficient to form fractures that extend into the formation, providing additional pathways through which the oil or gas can flow to the well. Unfortunately, water rather than oil or gas may eventually be produced by the formation through the fractures therein. To provide for the production of more oil or gas, a fracturing fluid may again be pumped into the formation to form additional fractures therein. However, the previously used fractures first must be plugged to prevent the loss of the fracturing fluid into the formation via those fractures.
In addition to the fracturing fluid, other fluids used in servicing a wellbore may also be lost to the subterranean formation while circulating the fluids in the wellbore or otherwise placing fluids in the wellbore. In particular, the fluids may enter and be “lost” to the subterranean formation via lost circulation zones (LCZs) for example, depleted zones, zones of relatively low pressure, lost circulation zones having naturally occurring fractures, weak zones having fracture gradients exceeded by the hydrostatic pressure of the drilling fluid, and so forth. As a result, the service provided by such fluid is more difficult to achieve. For example, a drilling fluid may be lost to the formation during drilling operations, resulting in the circulation of the fluid in the wellbore being terminated and/or too low to allow for further drilling of the wellbore. Such conditions may be referred to as partial or complete loss of circulation or lost circulation. Also, a secondary cement/sealant composition may be lost to the formation as it is being placed in the wellbore, thereby rendering the secondary operation ineffective in maintaining isolation of the formation. Conventional solutions to preventing loss of wellbore fluids to an LCZ involve forming a viscous mass in the LCZ. This may be accomplished by mixing a relatively small volume of slurry or “pill” made up of insoluble swellable particles in a water-based fluid and pumping the pill into the wellbore to spot it, or place it, at the LCZ, where the particles swell to form a viscous aggregate mass that obstructs the LCZ. The slurry pill may contain inhibitors, such as salts, that slow the swelling of the swellable particles in the pill to prevent premature swelling which could plug the conduit to the LCZ. Frequently, the viscous masses are easily deformable and may breakdown under fluid pressure, thereby allowing reestablishment of a fluid flow channel within the LCZ. Accordingly, an ongoing need exists for more effective compositions and methods of blocking the flow of fluid through LCZs in subterranean formations.